Method of controlling shrinkage of castings



Oct. 23, 1951 w. c. SLINKARD ET AL 2,572,064

METHOD OF CONTROLLING SHRINKAGE 0F CASTINGS Filed Nov. 22, 1948 2SHEETS-SHEET 1 INVENTORS w.- c. SLINKARD F. L. DUNCAN ATTORNEY Oct. 23, 1951 w. c. SLINKARD ET AL 2,572,064

METHOD OF CONTROLLING SHRINKAGE OF CASTINGS Filed Nov. 22. 1948 2 SHEETS- SHEET 2 INVENTORS W. C. SLINKARD AND 2] F. L. DUNCAN ATTORNEY Patented Oct. 23, 1951 2,572,064 I C E METHOD OF CONTROLLING SHRINKAGE OF CASTINGS William O. Slinkard, Chula Vista, and Frank L. Duncan, San Diego, Calif., assignors to Solar Aircraft Company, San Diego, Calif., a corporation of California Application November 22, 1948, Serial No. 61,326

2 Claims.

This invention relates to the art of makin castings in sand molds, and is particularly useful in connection with the casting of dies for use in drop hammer or press operations.

An object of the invention is to control shrinkage of castings cast in sand molds and to reduce distortion resulting from unequal shrinkage.

In making relatively large castings in sand molds, particularly with some casting metals and alloys, it is found that unequal shrinkage occurs during cooling of-the casting in the mold. This has created difficulty, particularly in the casting of mating dies for use in hammer or press forming operations. The distortion has been found to be particularly severe where there is a concave portion of the casting filled with a sand core. It appears that, as a result of the heat from the hot metal, the sand core becomes extremely hard and unyielding so that it prevents normal contraction of that portion of the casting surrounding it. However, even when neither of two mating dies cast in sand molds contained shallow cavities in which the sand of the mold could become compacted, it has been found that the two dies were often relatively distorted during the cooling operation, so that their fit was very poor.

In accordance with the present invention, this unequal contraction is largely prevented by removing the castings from the sand molds as soon as they have cooled enough to retain their shape, and permitting them to complete their cooling down to room temperature exterior of the said mold. In many instances it is found advisable after removing the hot castings from the sand molds to bury them in loose sand to produce a more uniform cooling throughout.

Further in accordance with the invention it has been found that much improvement in a pair of mating dies can be had by removing both dies from their molds as soon as they have solidified enough to be handled and then placing them in mating relation with each other to complete their cooling down to room temperature. During this cooling operation the two mated dies can preferably be buried in loose, sand to further equalize the temperatures throughout. It appears that although some shrinkage of the casting occurs while it is cooling to a temperature at which it can be removed from the mold, most of the shrinkage occurs during the cooling from the temperature of solidification down to room temperature. It further appears that the slight amount of shrinkage occurring during cooling to a safe handling temperature does not exceed the elastic limit of the metal, so that the casting recovers its normal size when the inhibiting effect of hard packed sand of the mold is removed therefrom. In the case of mating dies of large dimensions, it appears that, where the dies are cooled separately, relatively unequal contractions of the working faces of the dies occur because of the varying masses of material adjacent different portions of the die face. However, if the two mating dies are placed together in mating relation before shrinkage occurs, then the only por-- tions of the dies that are exposed are the exterior faces which are usually flat or round and, Since the working; faces are completely within the external exposed. surfaces, they maintain a substantially uniform, temperature gradient throughout during the coolrelatively regular in shape.

ing process.

A full understanding of the invention may be had from the following description describing certain particular practices in accordance with, the invention with reference to the drawing, in

which Fig. 1 is a perspective view of a sand mold containing a casting that has been poured in the.

cavity defined by the mold;

Fig. 2 is a section in the plane II-II of Fig. 1. showing how the sand becomes hardened adjacent the casting by the heat emanating there--- from tion;

Figs. 7 and 8 show the two complementary dies;

removed from the molds; and

Fig. 9 shows the two complementary dies placed together in mating relation and buried in loosesand for completion of the cooling to room temperature.

Figs. 1 to 4 inclusive show the application of the present process to the cooling of a simple casting having a cavity therein. Fig. 1 shows the casting II] in a conventional sand mold I I. It will be observed from Fig. 2 that this casting l0 has a concavity [3 in one face, this concavity being surrounded by an annular rim i4. Afterv the molten casting metal has been poured into the mold, the metal cools by radiation and conduction of heat into the sand of the casting. This raises a portion N5 of the sand immediately adjacent the casting to a relatively high temperature, and it is frequently found to bake the sand into a hard substantially incompressible formathe sand mold, of from .025 inch to .109 inch.

This distortion can be substantially completely avoided in accordance with the present invention by removing the casting 10 from the mold as 

